In recent years, the so-called amorphous silicon electrophotographic photoreceptor having a layer whose principal component is amorphous silicon as the photosensitive layer has been the subject of attention. This is because the amorphous silicon material has the potential of fundamentally improving the life factors of the conventional electrophotographic photoreceptors, and its application to an electrophotographic photoreceptor makes it possible to obtain a sufficiently hard, thermally stable and long-lived electrophotographic photoreceptor with electrically stabilized reproducibility. Consequently, there have been proposed various kinds of amorphous-silicon-based electrophotographic photoreceptors.
Of these, an amorphous electrophotographic photoreceptor having the so-called separated function type photosensitive layer in which the photosensitive layer is separated into a charge generating layer generates electric charge carriers under the irradiation of light and a charge transporting layer which permits an efficient injection of the charge carriers generated in the charge generating layer and an efficient movement of the charge carriers, has been proposed as an excellent photoreceptor. As the charge transporting layer for such a separated function type amorphous silicon electrophotographic photoreceptors use has been made, for example, of a layer obtained by decomposing a gaseous mixture in glow discharge. The mixture comprises a gaseous silane compound such as silane or disilane, a gas containing carbon, oxygen or nitrogen, and a gas containing a trace of group III or group V elements (for example, phosphine or diborane) to form an amorphous silicon film containing the above-mentioned elements to a thickness in the range of 5-100 .mu.m.
In general, in an electrophotographic photoreceptor that is functionally separated into a charge transporting layer and a charge generating layer, its charging property is affected by the characteristics of the charge transporting layer, which has a thickness that is the largest among the various layers in the photosensitive layer. However, the chargeability of the electrophotographic photoreceptor using a charge transporting layer of hydrogenated amorphous silicon film obtained by the glow-discharge decomposition of a silane compound as exemplified above is still at an insufficient by low level on the order of about 30 V/.mu.m or smaller. Further, its rate of dark decay, although different depending upon the conditions of use, is in general at least about 20%/sec, which is extremely high. For this reason, an electrophotographic photoreceptor using such an amorphous-silicon-based charge transporting layer was either limited in its use to systems with relatively fast operation or required a specific developing system bceause it is not possible to obtain a sufficiently high charge potential. In order to raise the charge potential, the thickness of the charge transporting layer may be increased. However, this leads to an extremely high cost for the photoreceptor by requiring an increase in the manufacturing time, and further by the induced reduction of the gain in the ordinary manufacturing method due to the increase in the probability of generating film defects accompanying the manufacture of thick films.
The applicants of the present invention had previously proposed an electrophotographic photoreceptor that uses an aluminum oxide film as a charge transporting layer. However, it was later found in this electrophotographic photoreceptor that there are sometimes cases in which cracks are generated in the charge transporting layer. When cracks are generated in the charge transporting layer, the charging properties of the electrophotographic photoreceptor becomes nonuniform, which leads to the impossibility of obtaining pictures of satisfactory quality. With a further investigation at that point, the applicants discovered that when an aluminum oxide film is formed by a specific method of manufacture according to the present invention, no cracks occurred in the aluminum oxide film.